Document Python version selection

Clarifies how TofuPilot picks the Python version for a procedure so users avoid "No interpreter found" and package-incompatibility errors.

• Added a Python version section explaining selection happens at the minor level (3.12), not the patch (3.12.4), with supported minors 3.10 to 3.14.
• Documented the gotcha for packages limited to older minors and how to cap the upper bound in requires-python.
• Noted that air-gapped stations can only use Python minors already installed locally, since provisioning a new minor requires fetching the interpreter.